She will later note that the paper-towel dispenser in the women's bathroom is empty.

Back at her apartment, she bangs out a detailed report for the restaurant's corporate managers. Tonya  not her real name  is a "mystery shopper," and her caseload has never been bigger.

The recent college grad has been checking into hotels, posing as a potential clothing buyer  even crashing wedding receptions to spy on caterers.

Tonya works for Boston-based Data Quest Investigations Ltd., one of about 500 mystery-shopping firms that now exist in the United States  a 25 percent increase from three years ago.

Mystery shoppers are independent contractors who choose from a range of assignments from one or more mystery-shopping firms. These companies act as middlemen, training competent shoppers to provide feedback to service providers.

Each week, the firms send out as many as 500,000 amateur detectives like Tonya to rate firms on anything from cleanliness to customer service, management, and product quality.

Fueling this $500 million industry is an awareness among retailers that service is increasingly important to consumers who have become frustrated by retailers' inaccessible bureaucracies and automated help lines.

"Especially in times like these, when people have less discretionary money, companies realize they have to do everything they can to differentiate themselves and keep customers coming back," says Rodney Moll, president of the San Diego-based mystery-shopping firm TrendSource.

His company, whose clients include Taco Bell, Coca-Cola, and DaimlerChrysler, notched a 40 percent increase in revenue last year.

One of the most recent companies to use mystery shoppers: McDonald's. The fast-food giant hopes the practice will help revive its sluggish sales and dismal customer-service ratings.

Other fast-food purveyors  such as Taco Bell, which has used undercover auditors from TrendSource for five years now  say they notice a big difference.

"We have thousands of franchises.... We need an extra pair of eyes to detail what's really going on in the field," says Caroline Anawati, a Taco Bell spokeswoman.

Their mystery shoppers not only place orders and observe service, they also whisk the meals out to their vehicles for a closer inspection.

One result of all the attention: The company has slashed 47 seconds off its average drive-through wait time, says Ms. Anawati.

"The quality of our products and service has also gone up since we've used mystery shoppers.... We need to make sure the customer's experience is the exact same quality [at all franchise outlets]," she adds.

But as the number of mystery-shopping firms multiplies, so have debates over their use.

Noe Cabrales manages a Burger King franchise in Los Angeles. This month his mystery-shopping report tanked to 79 percent from his usual score of 100. The store was marked down for a long wait at the drive-through.

But he insists the mystery shopper must have been last in a long line of cars, behind a man he recalls having ordered seven combos  plus extra onion rings.

"The mystery shopper didn't know about that huge order. It's not fair," he says, adding that the reports are closely tied to his pay increases.

But mystery-shopping firms insist that their methods have improved through the years.

"The industry has grown from having a 'gotcha' mentality to one of providing a positive incentive for employees," says Mr. Moll.

It's now common, he says, for mystery shoppers to give deserving employees small rewards, such as movie tickets, on the spot. Employees who receive low marks, Moll adds, are more likely to be given extra training than to be dismissed.

Another mystery-shopping trend: the increased use of high-tech gadgets for snooping.

Krispy Kreme Doughnut Corp., for example, has mystery shoppers use tiny digital cameras to detect how clean stores' restrooms are, or how evenly jelly is inserted into its doughnuts.

The images are downloaded onto a laptop and sent to the company along with a Web-based evaluation form.

The result: telling, full-color accounts that land on the store managers' desk in 48 hours.

"Now we're able to spend less time debating over whether the mystery shopper was accurate or even showed up, and more time on how we can improve," says Steve Anderson, director of Krispy Kreme's "customer experience" department.

But such methods have drawn the ire of privacy-rights advocates and labor unions.

"There has to be some reason for this kind of surveillance to take place, but most often the process is put in place just in case something is going wrong," says Sarah Andrews, research director at the Washington, D.C.-based Electronic Privacy Information Center. "That doesn't foster a healthy working environment."

Similar concerns have been made by people within the mystery-shopping industry itself.

"I wouldn't want to feel like Big Brother was watching me through a small camera in my next customer's hat," says Mark Csordos, former founder of C&S Mystery Shoppers, based in New Jersey. C&S does its sleuthing by low-tech means.

Job hunters: Want to stake out a takeout?

How would you like to shop till you drop  and get paid for doing so? Mystery shoppers, who pose as customers to audit product and service quality, are in demand.

The undercover employees are hired by one of the estimated 500 mystery-shopping firms in the United States. These firms are contracted by a range of retail companies.

Restaurants and retailers are the heaviest users of such sleuths, but a growing number of hotels, airlines, real-estate agencies  even daycare centers and doctors' offices  are signing up.

In the case of apartments or clothing, the mystery shopper typically feigns interest to see how a real customer might be treated. But the job comes with real perks: Shoppers are reimbursed for the food or hotel stays they purchase.

One of the hardest parts of the job, says "Tonya," a part-time secret shopper in Boston, is keeping her cover  especially from friends. When asked what she does, "I just tell people about my other part-time job at [a nonprofit cultural center in Boston]," she says.

Some of her other covert characteristics: "I jot notes for myself on crossword puzzles and Palm Pilots so as not to arouse suspicion from employees," she says. She might write that it took 5 minutes and 38 seconds to receive her mesclun salad, or that her waiter knew the menu well.

One of her hardest assignments, she says, was crashing a wedding reception for a hotel to snoop on its servers.

"I couldn't say a word to anyone  or even make eye contact  because if a conversation were struck up, they would quickly know I didn't know the bride or groom  let alone their names," she laughs.

Tonya says she also loves the flexibility provided by the profession  which is mostly made up of students, part-time professionals, and mothers with kids in tow. "But it's also hard work to keep track of so many details, and be on constant alert," she says.

It helps to have an ironclad memory, a background in retail or customer service, and a knowledge of the firm for which one is snooping.

Judith Rappold can vouch for that last one. She says she once had to mystery-shop for a mattress company  "and learn dozens of different types of foam, quilting, springs ... you could really go crazy learning it all."

In comparison with other jobs, the overhead needed to get started as a mystery shopper is minimal. A pen and pad about covers it.

"This is a great business because the investment is so low," says Ms. Rappold, who started Business Resources in 1984 from a home office in Austin, Texas.

Tonya earns about $15 per shopping trip from the Boston firm Data Quest Investigations Ltd. If she worked 40 hours a week, that would translate to about $30,000 a year  not counting the free meals or weekend resort stays.